1. Field of Invention
The present invention relates to a metal-oxide-semiconductor structure and corresponding method of manufacture. More particularly, the present invention relates to an air gap semiconductor structure and corresponding method of manufacture.
2. Description of Related Art
Following the rapid reduction of semiconductor line width, the accompanied increase in resistor-capacitor (RC) time delay has greatly reduced the operating speed of integrated circuits. To reduce RC time delay, methods that can lower resistance is frequently adopted. The most common trend now is to replace conventional aluminum wires by copper wires.
A second way of reducing RC time delay is to reduce capacitance between conductive wires in a multi-layer design. Conventional silicon dioxide is no longer versatile enough for this purpose because silicon dioxide has a relatively high dielectric constant. In general, low dielectric constant organic or inorganic material is used to form inter-metal dielectric layer. However, a medium having the lowest dielectric constant is air (a dielectric constant of 1). Therefore, air is ideal dielectric medium for lowering the capacitance of a capacitor.
Although air is the best dielectric material for lowering capacitance, overall mechanical strength of the device is reduced correspondingly. A weakened structure can have serious effect in various aspects of subsequent integrated circuit fabrication.
A device having air as a dielectric medium generally has air gaps between metallic lines. Air gaps will destabilize the semiconductor device and lead to structural deformation. Moreover, the air gaps are normally distributed all over the substrate. Although capacitor has low capacitance when air is used as a dielectric medium layer, poor heat conductivity of air often leads to a rapid accumulation of heat and a rise in temperature during operation.
Accordingly, one object of the present invention is to provide an air gap semiconductor structure and corresponding method of manufacture. The method includes forming a sacrificial polymer film over a substrate having metal lines thereon. A portion of the sacrificial polymer film is subsequently removed to form first spacers. A micro-porous structure layer is formed over the substrate and the metal lines and between the first spacers. A portion of the micro- porous structure layer is removed to form second spacers. The first spacers are removed by thermal dissociation to form air gaps. Finally, a dielectric layer is formed over the substrate and the metal lines and between the second spacers.
In this invention, air gaps are only formed on the sidewalls of the metal lines. Other sections of the substrate are covered by conventional dielectric material. With this arrangement, not only can RC delay be lowered to increase operating speed of integrated circuits, mechanical strength of the integrated circuit can also be maintained.
In addition, the micro-porous structure layer increases the diffusion efficiency of by-products generated during the dissociation of the sacrificial polymer film, especially in the region between neighboring metal lines where height/separation ratio is high.
Furthermore, since only spacer-like air gaps are formed on the sidewalls of the metal lines, heat generated during device operation can easily be channeled away through surrounding conductive layer. Hence, heat dissipation problem can be ignored.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.